It is very closely related to the Haggis, I often go on short shooting breaks to Scotland to shoot Scallion I prefer the Haggis shoots but the Scallion does fly a little closer to the ground giving a more testing shoot!!
It's just another name for what we call a spring onion or imature onion in the UK.
I believe 'scallion' is the word commonly used in the USA, The Caribbean and I think Ireland.